A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a scanning type lithographic apparatus, a patterning device (e.g. a mask) is carried by a patterning device support, also referred to as mask table or patterning device table. While generating a pattern on a target portion of a substrate, the patterning device support performs scanning movements along a line of movement, in a single scan direction or scanning in both (i.e. opposite) directions along the line of movement. When a reversal of direction takes place, the patterning device support is decelerated and accelerated between the successive scanning movements. Also, the patterning device support is accelerated and decelerated before and after each scanning movement in a specific direction. Conventionally, the scanning movements are made with constant velocity. However, the scanning movements may also at least partly be made with varying velocity, e.g. the movements including at least part of the deceleration and/or acceleration phases.
The patterning device support holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The patterning device support may include a frame or a table, for example, which may be fixed or movable as required. The patterning device support (and its control system) may ensure that the patterning device is at a desired position, for example with respect to the projection system.
The patterning device is coupled to the patterning device support through a clamp. Conventionally, the patterning device is coupled to the patterning device support through a vacuum clamp which may be implemented as one or more vacuum pads provided on the patterning device support, where at least a part of a circumferential area of the patterning device is held onto the vacuum pads. By the clamp, a normal force between adjacent surfaces of the patterning device and the patterning device support is generated, resulting in a friction between contacting surfaces of the patterning device and the patterning device support. The vacuum pads may include one or more openings coupled to a gas discharge and supply system. Instead of a vacuum coupling between the patterning device and the patterning device support, other forms of couplings based on friction between the patterning device and the patterning device support are conceivable, such as electrostatic or mechanical clamping techniques to hold the patterning device against the patterning device support.
In an ongoing development, increasing throughput requirements placed on lithographic apparatus lead to increasing scanning velocities. Consequently, deceleration and acceleration of the patterning device support increase. In the deceleration and acceleration phases, increased inertia forces act on the patterning device support and on the patterning device.
It is known that inertia forces acting on the patterning device support and the patterning device may lead to slip of patterning device and patterning device support relative to each other. The slip usually is in the order of nanometers. For relatively low decelerations and accelerations, the slip has appeared to be low and approximately constant over time, changing its direction with each deceleration/acceleration phase. In such circumstances, the slip may be ignored if it is sufficiently low, or the slip may be compensated by suitably calibrating a positioning device controlling the position (and hence, the movement) of the patterning device support and/or the substrate stage.
However, with increasing decelerations and accelerations, the slip occurring between the patterning device and the patterning device support increases, and becomes variable and unpredictable. Factors influencing the amount of slip may include, but may not be limited to, a flatness and roughness of the surfaces of the patterning device and the patterning device support engaging each other, a humidity of the atmosphere(s) in which the patterning device and the patterning device support are handled, a contamination of the patterning device or the patterning device support, and a degree of vacuum when the patterning device is held on the patterning device support by vacuum pads. Thus, a calibration of the positioning device will not lead to a correct positioning of the patterning device support and/or the substrate stage under the circumstances of high inertia forces.
Not only the speed of movement and acceleration of the patterning device support may tend to increase, also, accuracy requirements on the lithographic apparatus may become more stringent. Therefore, slip of the patterning device becomes less tolerable, as slip of the patterning device may result in a position error of the pattern projected onto the substrate.
It has been proposed to provide mechanical solutions to avoid slip between the patterning device support and the patterning device, such as enhanced clamping force between the patterning device support and the patterning device and/or an optimized clamp design. Also it has been proposed to provide a patterning device pushing device which exerts a compensation force on a side of the patterning device to avoid slip between the patterning device and the patterning device. However, none of these solutions is capable of sufficiently avoiding the imaging errors, in particular overlay errors at higher acceleration levels of the patterning device support.
In another solution a feed-forward compensation controller was provided in which slip between patterning device and patterning device support was taken into account. However, the variation in the amount of slip at a certain acceleration level is unpredictable. As a result, feed-forward compensation may not provide a reliable compensation for the slip between patterning device and patterning device support.
In EP 1 918 777, the contents of which are herein incorporated by reference, it is proposed to provide a support position sensor to measure a position of the support relative to a first structure of the lithographic apparatus, and a patterning device position sensor to measure a position of the patterning device relative to a second structure of the lithographic apparatus. A control device is provided to determine a correlation between the position of the patterning device and the position of the support from the position measured by the support position sensor, the position measured by the patterning device position sensor, and mutual positions of the first and second structures. On the basis of this correlation, the amount of slip between the patterning device and the patterning device support is determined and compensated in the position control of the patterning device support.